The present invention relates to a lubricant which is useful in catheter-cannula assemblies. More particularly, the present invention relates to a lubricant which is useful in preventing time-temperature dependent increase in adhesion, which has been observed in some catheter assemblies in which a thermoplastic catheter is carried on the cannula with an interference fit between the tip of the catheter and the cannula. Still more particularly, the present invention relates to a lubricant useful with interference-fitted polyurethane catheters.
Catheter assemblies in which the catheter is formed from a thermoplastic polymer, in some cases, have been designed with an interference fit between the tip of the catheter and the cannula to facilitate skin and vein penetration and prevent catheter pull-back (i.e., the catheter pulling back or spreading on the cannula during insertion). Typically the interference fit is about 0.025 to 0.0625 mm (about 0.001 to 0.0025 inch).
Upon storage of interference fitted thermoplastic catheter assemblies for long periods of time or at elevated temperatures an excessive amount of adhesion develops between the catheter tip and the cannula. The catheter tends to soften and adhere to the surface of the cannula. Such high adhesion levels can develop that, after storage, the thermoplastic catheter is extremely difficult to remove from the cannula. In some cases, the adhesion is so great that the catheter cannot be removed without deformation. In any event, the catheter becomes so adhered to the cannula that it cannot be removed without causing significant additional discomfort or pain to the patient.
While it is known in the art to lubricate the cannula to facilitate insertion and removal of the catheter, previous lubricants have been less effective in preventing the aforementioned time-temperature dependent increase in adhesion. In particular, the organosiloxanes described in U.S. Pat. No. 3,574,673, such as Dow Corning MDX4-4159 Fluid, have been used as lubricants for hypodermic needles to provide needle point lubrication for skin and vein penetration. These organosiloxanes are copolymers of dimethylsiloxane and aminoalkylsiloxanes and moisture cure to a relatively soft and gelatinous film. These organosiloxanes are referred to as gelling siloxanes and, depending on ambient humidity, they require several days to fully cure. When they are applied to a hypodermic needle, partially cured, and inserted into an interference-fitting catheter, in the conventional manner, the silicone film is largely wiped up the needle and accumulates in a visible ring above the tip of the needle. This limits use of this material to very light coatings which will not accumulate in a ring. The very thin film which remains between the catheter tip and cannula does not effectively prevent the time-temperature dependent increase in adhesion. Thus, a significant disadvantage of gelling siloxanes is that a minimum of several hours is required to partially cure the coating before the cannula can be assembled with the catheter and heavier coatings which would be more effective in adhesion control cannot be used.